# "Equivalence" demonstrated: Canon 5D and Panasonic GX1

Started Apr 27, 2013 | Discussions
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Funny you mention it...

pavinder wrote:

...And, in terms of how this relates to IQ, I think the conclusion of your linked article sums it up really well.

"Each photographer must balance the operation of a system against its IQ potential not only in concert with the display size of the image, but also with both their skills in photography and post-processing, to decide what system best gets the job done for the type of photography that they do."

However we then will get into discussions of what constitutes "high IQ"!

Can we say "aesthetic IQ" and "technical IQ"?   (･_･)

...'cause that point is discussed, in detail, here:

http://www.josephjamesphotography.com/equivalence/#IQ

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Your examples do not demonstrate 'equivalence'
2

walkaround wrote:

There has been a lot of talk by a few people here about "total light", and how an f/1.4 lens on a m4/3 camera supposedly acts like an f/2.8 in regards to "light on the sensor", etc.

This is based on the ratio of the sensor sizes.

Because 135 is four times the area of FT, for the same exposure the 135 sensor collects four times as much light.  It is as simple as that -- try not to let other "explanations" muddy this.

Now, the 'equivalence' argument (which is theoretically sound, though would not likely occur precisely in reality for various reasons) builds on this with an assumption that the same total light (not exposure -- exposure is total light divided by sensor area, or light density) applied to any sensor will yield the same noise.

So, if this assumption is correct (and it should be -- sort of -- in theory: it gets complicated and further assumptions must be brought in) then the argument then proceeds that you should be able to raise the ISO on a larger sensor and obtain the same noise.

So, for your two shots we should be able to obtain 'equivalent photographs' with the following settings:

• GX1 50 mm @ f/1.4 1/250 ISO 160
• 5D 100 mm @ f/2.8 1/250 ISO 640 (-2 stops aperture, +2 'stop' ISO)

(Note that the ISO on the 5D is raised 'two stops' to counter the relative aperture being lowered two stops -- ISO isn't really measured in 'stops', thus my quote marks to appease the anal types.)

What you should get, in theory, is two 'identical'(ish) shots with the same characteristics -- framing (actually no, due to 3:2 v 4:3, but you should get the same angular coverage across the diagonal), DOF, and also apparent brightness and noise.

You will most likely find that this is not quite the case, due to the 5D being an older design and having a comparatively worse sensor.

However, if you ignore the fact that this doesn't really work in reality, it is a reasonably solid idea.  This is why you get people claiming that a given lens on FT is 'equivalent' to a lens with 2 stops less aperture on 135.

Refer to DPR using this concept when explaining the utility of the fast Sigma zoom on an APS-C body, and how that f/1.8 zoom 'equalises' APS-C with a 135 system using an f/2.8 zoom:

http://www.dpreview.com/previews/sigma-18-35-1-8

Sigma's choice of F1.8 as maximum aperture isn't a coincidence; it means that the lens will offer the same control over depth of field as an F2.8 zoom does on full frame. What's more, it will also offer effectively the same light-gathering capability as an F2.8 lens on full frame. By this we mean that it will be able to project an image that's just over twice as bright onto a sensor that's slightly less than half the area, meaning the same total amount of light is used to capture the image. This is important as it's a major determinant of image quality. Essentially it means that APS-C shooters will be able to use lower ISOs when shooting wide open in low light and get similar levels of image noise, substantially negating one of the key advantages of switching to full frame.

(My emphasis.)

Now, you have to bear in mind that unless noise is an important issue then this is something of an irrelevance (IMO).  My preference is to simply understand that a larger sensor will yield real benefits when the ISO must be raised quite high (and this changes: so my E-5 is pretty good through to ISO 1600, then falls off severely; where the E-M5 is good through to ISO 3200, then falls off less severely), or when you want a shallower DOF for a given focal length.

Oh, and also bear in mind that my caveats above with respect to 'equivalence' can also be ignored if you use the phrase "all else being equal".  This is what Joe (Great Bustard) prefers to use, and is perfectly fine provided you realise that this is not likely to be the case in reality.

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Re: "Equivalence" is not "Equivalence" - try a new terminology.

Great Bustard wrote:

Similarly to say "these 2 photos are visually equivalent but not technically equivalent" is far more helpful than to just argue about whether they are "equivalent" under some all-encompassing definition.

So, what are your parameters for "visually equivalent" and what are your parameters for "technically equivalent"?  I mean, next thing we know we're going to be talking all sorts of "equivalences" for size, weight, price, AF speed/accuracy, build, etc., etc., etc.

Visually equivalent - very simple.  And exactly what it says. They look the same.

Framing, perspective, what's in focus and what's not, what's light, what's dark, etc.

In other words if you showed someone the 2 photos, they would look and think "yes, it's the same photo".

I would think that this is, for most photographers, what's important.  It will answer the question "Can X camera take a shot that looks the same as Y camera?"

Technically equivalent - very simple.  They have the same quantitative technical details.

And this could of course be seperated into IQ-related details (noise levels, size, etc) and settings-related details (aperture, ISO, shutter speed, etc).

Clearly these settings-related details have more direct effect on the visual properties of the photo that the IQ-related details.  But (because they're quantitative, not qualitative) they're still distinct from the aesthetic details which determine visual equivalence.

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Re: "Equivalence" is not "Equivalence" - try a new terminology.
1

pavinder wrote:

Great Bustard wrote:

Similarly to say "these 2 photos are visually equivalent but not technically equivalent" is far more helpful than to just argue about whether they are "equivalent" under some all-encompassing definition.

So, what are your parameters for "visually equivalent" and what are your parameters for "technically equivalent"?  I mean, next thing we know we're going to be talking all sorts of "equivalences" for size, weight, price, AF speed/accuracy, build, etc., etc., etc.

Visually equivalent - very simple.  And exactly what it says. They look the same.

Framing, perspective, what's in focus and what's not, what's light, what's dark, etc.

In other words if you showed someone the 2 photos, they would look and think "yes, it's the same photo".

Well, that's basically Equivalence as I've defined it, then:  same perspective, framing, DOF, shutter speed (motion blur), and display size.

What's not included is: noise, resolution, dynamic range, color, etc., although these can also be accounted for with additional assumptions about the technology.  However, the five parameters I use for Equivalence are independent of the technology.

I would think that this is, for most photographers, what's important.  It will answer the question "Can X camera take a shot that looks the same as Y camera?"

Technically equivalent - very simple.  They have the same quantitative technical details.

And this could of course be seperated into IQ-related details (noise levels, size, etc) and settings-related details (aperture, ISO, shutter speed, etc).

Clearly these settings-related details have more direct effect on the visual properties of the photo that the IQ-related details.  But (because they're quantitative, not qualitative) they're still distinct from the aesthetic details which determine visual equivalence.

I dunno.  From where I'm sitting, I think "Equivalent" meaning same perspective, framing, DOF, shutter speed, and display size is pretty much the most "natural" use of the word with respect to photography.

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Re: "Equivalence" is not "Equivalence" - try a new terminology.

Great Bustard wrote:

pavinder wrote:

Great Bustard wrote:

Similarly to say "these 2 photos are visually equivalent but not technically equivalent" is far more helpful than to just argue about whether they are "equivalent" under some all-encompassing definition.

So, what are your parameters for "visually equivalent" and what are your parameters for "technically equivalent"?  I mean, next thing we know we're going to be talking all sorts of "equivalences" for size, weight, price, AF speed/accuracy, build, etc., etc., etc.

Visually equivalent - very simple.  And exactly what it says. They look the same.

Framing, perspective, what's in focus and what's not, what's light, what's dark, etc.

In other words if you showed someone the 2 photos, they would look and think "yes, it's the same photo".

Well, that's basically Equivalence as I've defined it, then:  same perspective, framing, DOF, shutter speed (motion blur), and display size.

Well 3 out of 5 is not quite the same.

Shutter speed and display size are quantitative, not qualitative.

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Re: "Equivalence" is not "Equivalence" - try a new terminology.
2

pavinder wrote:

Great Bustard wrote:

pavinder wrote:

Great Bustard wrote:

Similarly to say "these 2 photos are visually equivalent but not technically equivalent" is far more helpful than to just argue about whether they are "equivalent" under some all-encompassing definition.

So, what are your parameters for "visually equivalent" and what are your parameters for "technically equivalent"?  I mean, next thing we know we're going to be talking all sorts of "equivalences" for size, weight, price, AF speed/accuracy, build, etc., etc., etc.

Visually equivalent - very simple.  And exactly what it says. They look the same.

Framing, perspective, what's in focus and what's not, what's light, what's dark, etc.

In other words if you showed someone the 2 photos, they would look and think "yes, it's the same photo".

Well, that's basically Equivalence as I've defined it, then:  same perspective, framing, DOF, shutter speed (motion blur), and display size.

Well 3 out of 5 is not quite the same.

Shutter speed and display size are quantitative, not qualitative.

They're all quantative.  We can quantify perspective (subject-camera distance).  We can quantify framing (width and height of the scene on the focal plane).  We can quantify DOF (total distance front and back of the focal plane that is within critical focus).  In other words, they are all quantative just like shutter speed and display size.

Here, let me ask you a question:  would anyone disagree that 25mm f/2.8 1/100 ISO 400 with the 14=54 / 2.8 on an Olympus E1 is equivalent to the same settings with the 12-35 / 2.8 on an Olympus EM5?  I'm thinking no one would say they are not "equivalent", despite the fact that the noise, detail, dynamic range, etc., would all be quite different.

Thus, I see no reason not to use the word "equivalent" to describe photos that have the same perspective, framing, DOF, shutter speed, and display size even when other elements of IQ are not necessarily the same.

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Re: "Equivalence" demonstrated: Canon 5D and Panasonic GX1

I'd like to make some points sbout equivalence. I would define it as "Two photographs taken with different format cameras which share the same depth of field, shutter speed, total light on the sensor and exposure, and thus have the same compositional and noise characteristics."

1. Equivalence is only relevant in low light

If a scene is well lit, both cameras will be using ISO 100, and so they will not take equivalent pictures, because the larger format will have more total light, thus lower noise. At base ISO this will manifest itself as better colour depth and dynamic range.

2. Equivalence is only relevant when not using a tripod

For the same reasons as 1. - both will use lower ISO and different shutter speeds. This includes any situation in which the light is so low to necessitate using a tripod.

3. Equivalence is only relevant if you cannot light the scene yourself

If you can use flash or other lighting, and it is able to enable low ISO photography, then you are in the same situation as 1.

4. Equivalence is only relevant if depth of field is non-negotiable

In many real world low light situations, getting a cleaner shot using lower ISO is a higher priority than maintaining a deep depth of field. This could be because other parts of the image are too dark for it to matter whether they are in sharp focus, or because there is movement in the frame which makes a quick shutter speed a priority over depth of field.

So ultimately, comparing cameras based on equivalency means comparing them in this exact situation: medium-low light, no tripod, no flash and mandatory deep depth of field. Do you often take pictures in that exact situation? No, me neither.

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'Equivalent' can mean many different things
2

Great Bustard wrote:

They're all quantative.  We can quantify perspective (subject-camera distance).  We can quantify framing (width and height of the scene on the focal plane).  We can quantify DOF (total distance front and back of the focal plane that is within critical focus).  In other words, they are all quantative just like shutter speed and display size.

Here, let me ask you a question:  would anyone disagree that 25mm f/2.8 1/100 ISO 400 with the 14=54 / 2.8 on an Olympus E1 is equivalent to the same settings with the 12-35 / 2.8 on an Olympus EM5?

I would disagree.  The E-1 produces a very different photograph to the E-M5, even when using the same lens on both.

So I would say that such photos were "of the same subject" or even "the same photo, using two different cameras".  The point would actually be that they are not equivalent.  If they were, I could have saved a lot of money upgrading bodies to obtain a technically better photograph.

(Edit: OTOH, the E-1 may produce as perfectly good a result as the E-M5 for a given situation and end use.  It may be 'good enough'.)

I'm thinking no one would say they are not "equivalent", despite the fact that the noise, detail, dynamic range, etc., would all be quite different.

You have a very narrow idea of what 'equivalent' should mean -- or rather 'equivalent photographs', where you use 'equivalent' as a short-hand for that concept.

Because of this narrow idea, you then have trouble with the concept that a 50 mm f/2 lens on FT is equivalent to 100 mm f/2 on 135.  In this case, we'd be considering equivalent focal length in 135 terms: 100 mm EFL (where "EFL" is an 'industry standard' terminology using the 135 film system as a basis for comparison).

It is not in fact necessary to 'correct' such a statement to "50 mm f/2 lens on FT is equivalent to 100 mm f/4 on 135" if you are discussing EFL.  The advantage that a larger sensor has (or may have) is always 'present', and may be pointed out if relevant.

Thus, I see no reason not to use the word "equivalent" to describe photos that have the same perspective, framing, DOF, shutter speed, and display size even when other elements of IQ are not necessarily the same.

Once again, I suggest that you use 'equivalent photographs' as a clearer indication of what you are referring to in preference to your short-form usage of 'equivalent' or 'equivalence'.  This prevents confusion when, for example, you jump into a discussion comparing lenses where other 'equivalents' may already being discussed and essentially try to narrowly redefine the word 'equivalent'.  It is irrelevant to the point that you are making (about total light capture and sensor noise level) and may lead to unnecessary confusion and argument.

'Equivalent' does not have the narrow meaning that you may prefer it to have.

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Re: 'Equivalent' can mean many different things
1

boggis the cat wrote:

Great Bustard wrote:

They're all quantative.  We can quantify perspective (subject-camera distance).  We can quantify framing (width and height of the scene on the focal plane).  We can quantify DOF (total distance front and back of the focal plane that is within critical focus).  In other words, they are all quantative just like shutter speed and display size.

Here, let me ask you a question:  would anyone disagree that 25mm f/2.8 1/100 ISO 400 with the 14=54 / 2.8 on an Olympus E1 is equivalent to the same settings with the 12-35 / 2.8 on an Olympus EM5?

I would disagree.  The E-1 produces a very different photograph to the E-M5, even when using the same lens on both.

So I would say that such photos were "of the same subject" or even "the same photo, using two different cameras".  The point would actually that they are not equivalent.  If they were, I could have saved a lot of money upgrading bodies to obtain a technically better photograph.

For the record, you are saying that a photo of a scene taken at 25mm f/2.8 1/100 ISO 400 on an Olympus E1 is *not* equivalent to a photo of the same scene also taken at 25mm f/2.8 1/100 ISO 400 on an EM5.

Noted.

I'm thinking no one would say they are not "equivalent", despite the fact that the noise, detail, dynamic range, etc., would all be quite different.

You have a very narrow idea of what 'equivalent' should mean -- or rather 'equivalent photographs', where you use 'equivalent' as a short-hand for that concept.

Because of this narrow idea, you then have trouble with the concept that a 50 mm f/2 lens on FT is equivalent to 100 mm f/2 on 135.  In this case, we'd be considering equivalent focal length in 135 terms: 100 mm EFL (where "EFL" is an 'industry standard' terminology using the 135 film system as a basis for comparison).

Wait, wait, wait -- you just said above that 25mm f/2.8 on an E1 is not equivalent to 25mm f/2.8 on an EM5.  So how is it that 50mm f/2 on 4/3 "equivalent to" 100mm f/2 on FF?

It is not in fact necessary to 'correct' such a statement to "50 mm f/2 lens on FT is equivalent to 100 mm f/4 on 135" if you are discussing EFL.

It's not necessary to mention the f-ratio at all if you are only discussing EFL.

The advantage that a larger sensor has (or may have) is always 'present', and may be pointed out if relevant.

If 50mm on 4/3 is "equivalent to" 100mm on FF because it has the same EFL, then f/2 on 4/3 is "equivalent to" f/4 on FF because it has the same aperture (entrance pupil) diameter for the same EFL (50mm / 2 = 100mm / 4 = 25mm) which will result in not only the same DOF, but the same total amount of light projected on the sensor for a given shutter speed, which, in turn, will result in the same noise for equally efficient sensors.

In a word -- "Equivalent".

Thus, I see no reason not to use the word "equivalent" to describe photos that have the same perspective, framing, DOF, shutter speed, and display size even when other elements of IQ are not necessarily the same.

Once again, I suggest that you use 'equivalent photographs' as a clearer indication of what you are referring to in preference to your short-form usage of 'equivalent' or 'equivalence'.

Equivalent settings produce equivalent photos.

This prevents confusion when, for example, you jump into a discussion comparing lenses where other 'equivalents' may already being discussed and essentially try to narrowly redefine the word 'equivalent'.  It is irrelevant to the point that you are making (about total light capture and sensor noise level) and may lead to unnecessary confusion and argument.

'Equivalent' does not have the narrow meaning that you may prefer it to have.

Well, first you say that 25mm f/2.8 1/100 ISO 400 on an E1 is not "equivalent to" 25mm f/2.8 1/100 ISO 400 on an EM5, then you say that 50mm f/2 on 4/3 is "equivalent to" 100mm f/2 on FF, so I'm thinking that if anyone is causing "unnecessary confusion"...

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I guess you could say...
1

KenBalbari wrote:

tko wrote:

Equivalence isn't about exposure. Everyone knows any sensor with the same exposure values has the same BRIGHTNESS. That is the purpose of exposure - to give the same brightness for any camera. The sole reason for it's existence.

Only if the sensitivity of the films or sensors are the same, if you want to be technical.

Of course, in photographic slang, it has become acceptable to include ISO as part of "exposure" (as in the "exposure triangle") in which case it does refer to image brightness.  In that case though, Great Bustard's version of full equivalence then does also produce equal exposure.

It's only when using the technically correct version of "exposure" that he has to say exposure is different.  But image brightness is the same; there is no "visable" difference in exposure if you boost the ISO until image brightness is the same when total light is the same.

[And I know you understood this, but I got the impression some may have been confused by it.]

...the exposures are "equivalent".  That is, f/2.8 1/100 ISO 400 on mFT puts the same total amount of light on the sensor and has the same brightness as f/5.6 1/100 ISO 1600 on FF, even though the actual exposures are two stops apart, but the effect of the exposures is the same, just as the effect of 25mm on mFT has the same effect as 50mm on FF, and f/2 on mFT has the same effect as f/4 on FF.

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Re: Your examples do not demonstrate 'equivalence'

boggis the cat wrote:

(Note that the ISO on the 5D is raised 'two stops' to counter the relative aperture being lowered two stops -- ISO isn't really measured in 'stops', thus my quote marks to appease the anal types.)

What is ISO "really measured" in, then?

What you should get, in theory, is two 'identical'(ish) shots with the same characteristics -- framing (actually no, due to 3:2 v 4:3, but you should get the same angular coverage across the diagonal), DOF, and also apparent brightness and noise.

You will most likely find that this is not quite the case, due to the 5D being an older design and having a comparatively worse sensor.

However, if you ignore the fact that this doesn't really work in reality, it is a reasonably solid idea.

In reality, it's pretty freakin' close for sensors of the same generation.

Now, you have to bear in mind that unless noise is an important issue then this is something of an irrelevance (IMO).

There's still DOF.  So I guess you're saying that if noise and DOF (which necessarily means the corners will be rendered significantly different for the vast majority of scenes) don't matter, then, sure.  May as well just use a compact then, right?

My preference is to simply understand that a larger sensor will yield real benefits when the ISO must be raised quite high (and this changes: so my E-5 is pretty good through to ISO 1600, then falls off severely; where the E-M5 is good through to ISO 3200, then falls off less severely), or when you want a shallower DOF for a given focal length.

Each stop decrease in total light reaching the sensor results in a 41% increase in photon noise.  However, as the light gets dimmer and dimmer, the read noise (the additional noise added by the sensor and supporting hardware) begins to become dominant, so, at a certain light level, the noise in the photo is dominated by the read noise.

Oh, and also bear in mind that my caveats above with respect to 'equivalence' can also be ignored if you use the phrase "all else being equal".  This is what Joe (Great Bustard) prefers to use, and is perfectly fine provided you realise that this is not likely to be the case in reality.

All else is never equal.  But "all else" can often be accounted for.  It's like gravity -- for the most part, we only need to worry about the earth.  But if we want to explain the tides, we have to include the moon.  If we want to explain spring tides, we need to include the sun.  Etc., etc., etc.

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Re: Your examples do not demonstrate 'equivalence'
1

Great Bustard wrote:

boggis the cat wrote:

(Note that the ISO on the 5D is raised 'two stops' to counter the relative aperture being lowered two stops -- ISO isn't really measured in 'stops', thus my quote marks to appease the anal types.)

What is ISO "really measured" in, then?

It's a proxy for sensitivity (or was, in film times).  It relates the exposure to the resulting image density for a given film (rated in ISO or an equivalent scale).

Unitless, I guess.

The point being that you need to factor exposure parameters (aperture and shutter speed) and sensitivity (ISO) to determine a "correctly exposed" photograph.  Use too high an ISO and it will be over-saturated; too low and it will be too dark (and possibly have excessive apparent noise).

Now, since you can start from a correctly exposed photograph of, for example, f/2 1/125" at ISO 100 then change the aperture to f/2.8 (-1 stop) and compensate by either changing the shutter speed to 1/64" or the ISO to 200 it can be useful to consider ISO changes in terms of a 'stop', even though that isn't technically accurate.

Indeed, I tend to consider 'exposure' to mean both exposure proper and ISO, even though that is completely incorrect from a technical point of view -- ISO is not an exposure parameter at all.  This is when I am thinking in terms of a correctly exposed photograph -- so, really, 'exposure' would be my short-form thinking for 'how I want this photograph to look' (generally, balanced or 'correctly exposed').

What you should get, in theory, is two 'identical'(ish) shots with the same characteristics -- framing (actually no, due to 3:2 v 4:3, but you should get the same angular coverage across the diagonal), DOF, and also apparent brightness and noise.

You will most likely find that this is not quite the case, due to the 5D being an older design and having a comparatively worse sensor.

However, if you ignore the fact that this doesn't really work in reality, it is a reasonably solid idea.

In reality, it's pretty freakin' close for sensors of the same generation.

Well, you admit that your 'equivalent photographs' concept ignores many image quality parameters.  The 5D and GX1 are not very close so I would expect the results to be skewed against the 5D.

Now, you have to bear in mind that unless noise is an important issue then this is something of an irrelevance (IMO).

There's still DOF.  So I guess you're saying that if noise and DOF (which necessarily means the corners will be rendered significantly different for the vast majority of scenes) don't matter, then, sure.  May as well just use a compact then, right?

DOF is a fairly obvious difference.

Also, I am still wanting to pick up a Panasonic FZ200 to -- shockingly enough -- take photographs with.  It must amaze Panasonic that there are still people stupid enough to want a different set of compromises in some cases and not all want to use "full frame" (aka 135 format) for every purpose.

Have you considered writing a spy novel?  You could improve it by making the spy use a 135 system instead of one of those entirely useless tiny spy-camera things to copy the secret documents.

My preference is to simply understand that a larger sensor will yield real benefits when the ISO must be raised quite high (and this changes: so my E-5 is pretty good through to ISO 1600, then falls off severely; where the E-M5 is good through to ISO 3200, then falls off less severely), or when you want a shallower DOF for a given focal length.

Each stop decrease in total light reaching the sensor results in a 41% increase in photon noise.  However, as the light gets dimmer and dimmer, the read noise (the additional noise added by the sensor and supporting hardware) begins to become dominant, so, at a certain light level, the noise in the photo is dominated by the read noise.

Oh, and also bear in mind that my caveats above with respect to 'equivalence' can also be ignored if you use the phrase "all else being equal".  This is what Joe (Great Bustard) prefers to use, and is perfectly fine provided you realise that this is not likely to be the case in reality.

All else is never equal.  But "all else" can often be accounted for.  It's like gravity -- for the most part, we only need to worry about the earth.  But if we want to explain the tides, we have to include the moon.  If we want to explain spring tides, we need to include the sun.  Etc., etc., etc.

Sure, but you can't explain tidal behaviour at a given location purely with reference to the tidal pull of the Moon and Sun (and any other orbiting bodies you may care to include if you like meaningless precision).

This is not to say that the general theory is useless, of course.  Someone could have saved Bill O'Reilly some embarrassment if they'd bothered to explain the general gist of it to him when he was a child, for example.

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Re: 'Equivalent' can mean many different things
1

Great Bustard wrote:

boggis the cat wrote:

Great Bustard wrote:

They're all quantative.  We can quantify perspective (subject-camera distance).  We can quantify framing (width and height of the scene on the focal plane).  We can quantify DOF (total distance front and back of the focal plane that is within critical focus).  In other words, they are all quantative just like shutter speed and display size.

Here, let me ask you a question:  would anyone disagree that 25mm f/2.8 1/100 ISO 400 with the 14=54 / 2.8 on an Olympus E1 is equivalent to the same settings with the 12-35 / 2.8 on an Olympus EM5?

I would disagree.  The E-1 produces a very different photograph to the E-M5, even when using the same lens on both.

So I would say that such photos were "of the same subject" or even "the same photo, using two different cameras".  The point would actually that they are not equivalent.  If they were, I could have saved a lot of money upgrading bodies to obtain a technically better photograph.

For the record, you are saying that a photo of a scene taken at 25mm f/2.8 1/100 ISO 400 on an Olympus E1 is *not* equivalent to a photo of the same scene also taken at 25mm f/2.8 1/100 ISO 400 on an EM5.

Correct.

Furthermore, going to ISO 1600 will result in even less 'equivalent' photographs.

Noted.

Also note that 'equivalent' does not have a specific meaning.

I'm thinking no one would say they are not "equivalent", despite the fact that the noise, detail, dynamic range, etc., would all be quite different.

You have a very narrow idea of what 'equivalent' should mean -- or rather 'equivalent photographs', where you use 'equivalent' as a short-hand for that concept.

Because of this narrow idea, you then have trouble with the concept that a 50 mm f/2 lens on FT is equivalent to 100 mm f/2 on 135.  In this case, we'd be considering equivalent focal length in 135 terms: 100 mm EFL (where "EFL" is an 'industry standard' terminology using the 135 film system as a basis for comparison).

Wait, wait, wait -- you just said above that 25mm f/2.8 on an E1 is not equivalent to 25mm f/2.8 on an EM5.  So how is it that 50mm f/2 on 4/3 "equivalent to" 100mm f/2 on FF?

The clue is in that industry standard terminology: EFL -- "Equivalent Focal Length".

You can invent as many other 'equivalents' as you want.  In fact, you use a concept of 'equivalent photographs'.

It is not in fact necessary to 'correct' such a statement to "50 mm f/2 lens on FT is equivalent to 100 mm f/4 on 135" if you are discussing EFL.

It's not necessary to mention the f-ratio at all if you are only discussing EFL.

"EFL" is simply a ratio using the 135 system as a basis, so in some cases no you would not.

The advantage that a larger sensor has (or may have) is always 'present', and may be pointed out if relevant.

If 50mm on 4/3 is "equivalent to" 100mm on FF because it has the same EFL, then f/2 on 4/3 is "equivalent to" f/4 on FF because it has the same aperture (entrance pupil) diameter for the same EFL (50mm / 2 = 100mm / 4 = 25mm) which will result in not only the same DOF, but the same total amount of light projected on the sensor for a given shutter speed, which, in turn, will result in the same noise for equally efficient sensors.

In a word -- "Equivalent".

No.  You need to state what you mean by 'equivalent' -- in this case, 'equivalent photographs'.

Once you have established that as the 'equivalence' then you can use the short-hand provided it causes no confusion.

Thus, I see no reason not to use the word "equivalent" to describe photos that have the same perspective, framing, DOF, shutter speed, and display size even when other elements of IQ are not necessarily the same.

Once again, I suggest that you use 'equivalent photographs' as a clearer indication of what you are referring to in preference to your short-form usage of 'equivalent' or 'equivalence'.

Equivalent settings produce equivalent photos.

The point here is: 'Equivalent Photographs'.

This prevents confusion when, for example, you jump into a discussion comparing lenses where other 'equivalents' may already being discussed and essentially try to narrowly redefine the word 'equivalent'.  It is irrelevant to the point that you are making (about total light capture and sensor noise level) and may lead to unnecessary confusion and argument.

'Equivalent' does not have the narrow meaning that you may prefer it to have.

Well, first you say that 25mm f/2.8 1/100 ISO 400 on an E1 is not "equivalent to" 25mm f/2.8 1/100 ISO 400 on an EM5, then you say that 50mm f/2 on 4/3 is "equivalent to" 100mm f/2 on FF, so I'm thinking that if anyone is causing "unnecessary confusion"...

• The E-1 produces a very different photograph to the E-M5, even when using the same lens on both.
• [The] concept that a 50 mm f/2 lens on FT is equivalent to 100 mm f/2 on 135.  In this case, we'd be considering equivalent focal length in 135 terms: 100 mm EFL (where "EFL" is an 'industry standard' terminology using the 135 film system as a basis for comparison).

I see no reason for your confusion.

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Canon 5D vs Canon G7 : a better illustration
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Here's a test I made some years ago :

Canon 5d @ iso 1600 1/200 + Canon 70-200/4 @ 188mm f/22

Canon g7 @ iso 80 1/200 44,4mm f/4.8

So, what do we see ?

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There is no accepted definition of "equivalent images"
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I recently took all my cameras and shot some photos of the same subject from the same distance with them.

These were my settings:

Leica M9 and 35mm lens at f/8, 1/60s, ISO 6400 (2500 pushed 1.3 stops in Lightroom)

Sony NEX-5N and 24mm lens at f/3.2, 1/60s, ISO 1000

Fuji X-E1 and 24mm lens at f/5, 1/60s, ISO 2500

Olympus E-PM2 and 17mm lens at f/4, 1/60s, ISO 1600

I resized all of those to the same diagonal pixel dimension and showed them to a bunch of family and friends, asking the simple question "Which of these images look equivalent to you?"  When pressed to explain what I meant by equivalent, I offered the guidance "Similar in visual properties".

According to Joe's proposed definition of "equivalent images", the Leica, Fuji, and Olympus images were equivalent, and the Sony not equivalent.  Indeed, I could tell that the Sony image had less DOF than the others.  Yet everyone picked the Sony, Fuji, and Oly shots as equivalent, noting that the obvious difference in visual properties was how noisy the Leica image looked.  The second most common observation had to do with slight differences in color rendition.  No one commented on the DOF differences, including my wife who often complains about a photo being blurry when I use a narrow DOF.

To some people, a 35/1.4 on a Leica M9 doesn't produce an image equivalent to one made with a 35/1.4 on a D800.  To this guy, a Canon 35/1.4 on a 5D III doesn't produce an image equivalent to one made with a Sigma 35/1.4 on a D800.

Arguing about what is equivalent is like arguing about what is compact.  There is no accepted definition other than the common use as pertains to focal length equivalents.  There is a proposed definition of "equivalent image" but not an accepted one.

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... comparison continued (crops)
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Canon 5d iso1600

Canon g7 iso80

I see roughly the same amount of noise in the iso1600 image as in the iso80 image. As expected given the cropfactor of roughly 4,75.

Regards, P.

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Indeed, and that's why I prefer talk about equivalent settings.
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It is exactly these differences one sees when comparing images made using equivalent settings that make "equivalency" interesting, as they go beyond the obvious conclusions. The use of equivalent settings allows to really see difference in distortion, out of focus behaviour, sensor performance etc. That is why I prefer not to talk about equivalent images, lenses etc but merely about equivalent settings.

Regards, Pieter

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Re: "Equivalence" demonstrated: Canon 5D and Panasonic GX1

Aperture is really 'relative apeture'. Exposure should be the same. The 'equivalents' we are talikng about are field of view and depth of field.

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Re: ... comparison continued (crops)
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pidera wrote:

Canon 5d iso1600

Canon g7 iso80

I see roughly the same amount of noise in the iso1600 image as in the iso80 image. As expected given the cropfactor of roughly 4,75.

The results are quite similar.  The G7 result looks better to me, including less noise, but that could be due to the processing and / or better optical performance (with the 135 lens stopped down to f/22).

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I agree !
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Yes I agree with your observations that the g7 results look slightly better and that this is probably due to the camera and lens operating at settings for which it is optimised (unlike the FF setup at iso 1600 and f/22). Rgds, P.

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